The present invention relates to a measuring device for determining the evaporation rate of evaporation materials in vacuum systems, in particular for producing thin layers of exact thickness on substrates. The device has an electrically conducting housing with an opening which can be aligned with the evaporation material, and an electrode located inside the housing and insulated from the housing. This electrode is connected to an evaluation device for the evaporation rate.
A measuring device of the type described above is already known in the art, for example from the paper "An evaporation Rate Control System Employing a Heated Electrode Sensing Gauge" by M. H. Perkins, published in Trans. of the 8th Vacuum Sym. and 2nd Intern. Congress, 1961, pp. 1025 through 1030. The measuring device described there has the shape of a probe in the manner of an ionization manometer and is located directly in the main vapor stream. Aside from the fact that part of the vapor stream is shielded thereby, with a cold probe, vapors may condense on parts of the probe and in the course of time may make the probe useless. In order to eliminate influence on the measured result, anode and collector, during probe operation, are heated to a temperature of approximately 1000.degree. C. This requires not only a regulating system for the heater currents, but also suitable connecting cables which must withstand the high operation temperatures. The already known measuring probe requires eight separate connections requiring a correspondingly complicated design. Furthermore, there always is the danger that the heating conductor burns out so that a spare heating element is provided. Hence the already known device poses considerable requirements on the qualifications of the operating personnel during operation and regular monitoring of the known measuring probe.
From U.S. Pat. No. 3,168,418 there is already known a measuring device for the evaporation rate in vacuum evaporation systems which are preheated by vapor stream. The intensity of the vapor stream or the evaporation rate is measured by means of an additional electron beam gun by means of which the vapor is ionized in the measuring device. The resulting ion current is a measure of the evaporation rate and is used for controlling the heater power of the evaporator. The known arrangement is very complex in design and, because of the high-voltage supply to the electron beam gun and the requirement of special accelerating electrodes, requires a whole series of design measures which have a deleterious effect on the manufacturing and maintenance costs. However, a particular disadvantage is the fact that major parts of the measuring device are exposed to the main vapor stream which otherwise is narrowly limited by the measuring device. A comparable device with the inherent expenditure disclosed in U.S. Pat. No. 3,586,854, in which an electron beam oscillates in the vapor stream with a suitable catcher, is used for determining the evaporation rate.
It is, therefore, an object of the present invention to provide a measuring device of the initially described type which csn be located outside the vapor stream required for the evaporation, but which is free from wear and maintenance cost and does not require the heating of individual sections nor a high-voltage supply.
Another object of the present invention is to provide a measuring device of the foregoing character which is simple in design and construction, and may be fabricated economically.
A further object of the present invention is to provide a measuring device, as described, which has a long operating life.